Polarizing beam splitting system

ABSTRACT

A polarizing beam splitting system is described. The polarizing beam splitting system may include first and second prisms where the volume of the first prism is no greater than half the volume of the second prism. The first prism includes first and second surfaces and a light source may be disposed adjacent the first surface and an image forming device may be disposed adjacent the second surface. The first prism has a first hypotenuse and the second prism has a second hypotenuse. A reflective polarizer is disposed between the first and second hypotenuses.

BACKGROUND

Projection systems may include a light source and a polarization-rotating image-forming device which operates by rotating the polarization of light provided by the light source to produce an image. A polarizing beam splitter may be included to separate light having orthogonal polarization states.

SUMMARY

In some aspects of the present description, a polarizing beam splitting system including a reflective polarizer, first and second prisms, a light source and an image forming device is provided. The first prism has a first volume and includes a first face; a second face adjacent the first face, an angle between the first and second faces substantially equal to 90 degrees; and a first hypotenuse opposite the angle. The light source is disposed adjacent the first face and the image forming device is disposed adjacent the second face. The second prism is a right triangular prism having a second volume and having third and fourth faces and a second hypotenuse. The second hypotenuse is disposed adjacent the first hypotenuse. The first the first and second hypotenuses have substantially equal surface areas. The third face is opposite the first face and substantially parallel with the first face, and the fourth face is opposite the second face and substantially parallel with the second face. The reflective polarizer is disposed between the first and second hypotenuses, and the first volume is no greater than half the second volume.

In some aspects of the present description, a polarizing beam splitting system adapted to receive light from a light source and centered on a folded optical axis defined by an optical path of a central light ray emitted by the light source is provided. The polarizing beam splitting system includes an input surface substantially perpendicular to the optical axis, light entering the polarizing beam splitting system by passing through the input surface; a reflective polarizer having a largest lateral dimension d5, the optical axis having a length d1 between the input surface and the reflective polarizer; an output surface substantially perpendicular to the optical axis, the output surface having a length d3′ between the output surface and the reflective polarizer, light exiting the polarizing beam splitting system by passing through the output surface; and an imager face substantially perpendicular to the optical axis, the optical axis having a length d4 between the imager face and the reflective polarizer. One or both of d1 and d4 are less than d5/4.

In some aspects of the present description, a polarizing beam splitting system adapted to receive light from a light source and centered on a folded optical axis defined by an optical path of a central light ray emitted by the light source is provided. The polarizing beam splitting system includes an input surface substantially perpendicular to the optical axis, light entering the polarizing beam splitting system by passing through the input surface; a reflective polarizer, the optical axis having a length d1 between the input surface and the reflective polarizer; a first reflective component substantially perpendicular to the optical axis, the optical axis having a length d2 between the first reflective component and the reflective polarizer, the first reflective component being a tilted dichroic plate; a second reflective component substantially perpendicular to the optical axis, the optical axis having a length d3 between the second reflective component and the reflective polarizer; and an output face substantially perpendicular to the optical axis, light exiting the polarizing beam splitting system by passing through the output surface, the optical axis having a length d4 between the output surface and the reflective polarizer. One or both of d1 and d4 are less than a lesser of d2 and d3.

In some aspects of the present description, a polarizing beam splitting system adapted to receive light from a light source and centered on a folded optical axis defined by an optical path of a central light ray emitted by the light source is provided. The polarizing beam splitting system includes an input surface substantially perpendicular to the optical axis, light entering the polarizing beam splitting system by passing through the input surface; a reflective polarizer having a largest lateral dimension d5, the optical axis having a length d1 between the input surface and the reflective polarizer; a first reflective component substantially perpendicular to the optical axis, the optical axis having a length d2 between the first reflective component and the reflective polarizer, the first reflective component being a tilted dichroic plate; a second reflective component substantially perpendicular to the optical axis, the optical axis having a length d3 between the second reflective component and the reflective polarizer; and an output face substantially perpendicular to the optical axis, light exiting the polarizing beam splitting system by passing through the output surface, the optical axis having a length d4 between the output surface and the reflective polarizer. One or both of d1 and d4 are less than d5/4.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a polarizing beam splitter;

FIG. 2 is a side view of an illuminator;

FIG. 3 is a side view of an illuminator;

FIG. 4A is a side view of an illuminator;

FIG. 4B is a side view of a lens;

FIGS. 5A-5B are side views of an illuminator;

FIG. 5C is a top view of a reflective polarizer;

FIG. 6 is a schematic side view of an illuminator;

FIG. 7 is a schematic side view of an illuminator; and

FIG. 8 is a schematic illustration of a head mounted system.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanying drawings that forms a part hereof and in which various embodiments are shown by way of illustration. The drawings are not necessarily to scale. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense.

It is sometimes desired for a projection system to be compact. For example, hand-held pico-projectors and head mounted displays typically utilize compact projection systems. Such compact projectors may include a light source, a polarizing beam splitter, and a polarization-rotating image-forming device which operates by rotating the polarization of light provided by the light source to produce an image. The polarizing beam splitter often includes a reflective polarizer disposed between two right triangular prisms. Both prisms typically have the same volume and the polarizing beam splitter typically has opposing faces having the same area. According to the present description, illuminators are provided which can be more compact than traditional illuminators and which may be suitable for use in a projection system, for example. The illuminators may include a polarizing beam splitter having first and second prisms having differing geometries. For example, the first prism may have a volume substantially smaller than the second prism and/or may have faces having areas substantially smaller than corresponding areas of faces the second prism. Illuminators of the present description may allow a lens and/or a light source to be placed closer to the reflective polarizer than in traditional systems thereby achieving a more compact design. In some embodiments, the compact design may be achieved by utilizing a folded light path illuminator which provides a converging patterned light from an image forming device to a lens.

FIG. 1 is a schematic side view of polarizing beam splitter 100 including first prism 110, second prism 120, and reflective polarizer 130. First prism 110 includes first face 112, second face 114, first hypotenuse 116 and a portion 118 extending from the first and second faces 112 and 114. The first hypotenuse comprises a major surface of the portion 118. Second prism 120 includes third face 122, fourth face 124 and second hypotenuse 126. The second hypotenuse 126 is disposed adjacent the first hypotenuse 116 and the reflective polarizer 130 is disposed between the first hypotenuse 116 and the second hypotenuse 126. The polarizing beam splitter 100 may be part of a polarizing beam splitting system that includes the polarizing beam splitter 100 and that may include one or more additional optical components, such as one or more reflective components, for example. The polarizing beam splitter 100 may be part of an illuminator that includes the polarizing beam splitter 100 and that may include one or more additional optical components, such as a light source and/or an image forming device, for example. When used in an illuminator, first face 112 may be an input face disposed to receive light from a light source, second face 114 may be an output face, and fourth face 124 may be an imager face disposed adjacent an image forming device. In other embodiments, when used in an illuminator, third face 122 may be an input face disposed to receive light from a light source, second face 114 may be an output face, and fourth face 124 may be an imager face disposed adjacent an image forming device.

The second face 114 is adjacent the first face 112 with an angle α between the first and second faces 112 and 114. The angle α may be between 80 and 100 degrees, for example, or may be equal to or substantially equal to 90 degrees. The fourth face 124 is adjacent the third face 122 with an angle β between the third and fourth faces 122 and 124. The angle β may be between 80 and 100 degrees, for example, or may be equal to or substantially equal to 90 degrees. In some embodiments, the third face 122 is opposite the first face 112 and substantially parallel with the first face 112. In some embodiments, the fourth face 124 is opposite the second face 114 and substantially parallel with the second face 114. In some embodiments, the second prism 120 is substantially a right triangular prism. In some embodiments, the first the first and second hypotenuses 116 and 126 have substantially equal surface areas. Directions may be described as substantially perpendicular if an angle between the directions is within 10 degrees of 90 degrees (i.e., between 80 and 100 degrees). Such angles may be described as substantially equal to 90 degrees. Similarly, directions may be described as substantially parallel if an angle between the directions is no more than 10 degrees. In some embodiments, directions described as substantially parallel have an angle between the directions of no more than 5 degrees. In some embodiments, directions described as substantially perpendicular have an angle between the directions of between 85 and 95 degrees. Two faces may be described as substantially parallel or substantially perpendicular if directions normal to the two faces are substantially parallel or substantially perpendicular, respectively. Surfaces having areas within 15 percent of each other may be described as having substantially equal areas. In some embodiments, surfaces described as having substantially equal areas have areas that differ by no more than 10 percent, or no more than 5 percent.

An angle γ between the reflective polarizer and the fourth face 124 may be in a range of about 30 degrees, or about 40 degrees, to about 50 degrees, or to about 60 degrees, for example. As described elsewhere herein, an illuminator that includes the polarizing beam splitter 100 may have a folded optical axis having a segment substantially parallel with fourth face 124 and may have another segment substantially perpendicular to fourth face 124. An angle between the optical axis and the reflective polarizer may be equal to the angle γ or equal to 90 degrees minus γ. In some embodiments, an angle between the reflective polarizer and the optical axis is between about 40 degrees and about 60 degrees.

In some embodiments, the first prism 110 has a first volume, the second prism 120 has a second volume, and the first volume is no greater than about half the second volume. In some embodiments, the first volume is less than 35 percent, or less than 40 percent, or less than 50 percent, or less than 60 percent of the second volume.

In some embodiments, the first face 112 has a largest area (the total area of first face 112) that is less than about half of a largest area of the third face 122 (the total area of third face 122) and/or that is less than about half of a largest area of the fourth face 124 (the total area of fourth face 124). In some embodiments, the largest area of first face 112 is less than 60 percent, or less than 50 percent, or less than 40 percent, or less than 35 percent of the largest area of third face 122. In some embodiments, the largest area of first face 112 is less than 60 percent, or less than 50 percent, or less than 40 percent, or less than 35 percent of the largest area of fourth face 124. In some embodiments, the second face 114 has a largest area (the total area of second face 114) that is less than about half of a largest area of the third face 122 (the total area of third face 122) and/or that is less than about half of a largest area of the fourth face 124 (the total area of fourth face 124). In some embodiments, the largest area of second face 114 is less than 60 percent, or less than 50 percent, or less than 40 percent, or less than 35 percent of the largest area of third face 122. In some embodiments, the largest area of second face 114 is less than 60 percent, or less than 50 percent, or less than 40 percent, or less than 35 percent of the largest area of fourth face 124. In some embodiments, each of the largest area of the first face 112 and the largest area of the second face 114 is less than about half of a lesser of a largest area of the third face 122 and the largest area of the fourth face 124.

The prisms and the reflective polarizer in FIG. 1 are shown spaced apart for clarity of illustration. However, it should be understood that the various components could be in direct contact or attached through an optically clear adhesive, for example In some embodiments, the reflective polarizer 130 is bonded to one or both of the first and second prisms 110 and 120 through optically clear adhesive(s).

Reflective polarizer 130 may be any suitable type of reflective polarizer such as, for example, a polymeric multilayer reflective polarizer, a wire grid polarizer, a MacNeille reflective polarizer, or a cholesteric reflective polarizer. Suitable polymeric multilayer reflective polarizers are described, for example, in U.S. Pat. No. 5,882,774 (Jonza et al.), and U.S. Pat. No. 6,609,795 (Weber et al.) and include Advanced Polarizing Film (APF) available from 3M Company (St. Paul, Minn.).

The first and second prisms 110 and 120 may be made from any suitable materials such as, for example, glass, ceramics or optical plastics (e.g., polycarbonate, acrylates such as polymethylmethacrylate (PMMA), cyclic olefins, or other polymers). The first and second prisms can be made by any suitable process such as, for example, molding, machining, grinding and/or polishing. The material selected may have a low birefringence so that the polarization state is not significantly altered as light passes through the first or second prisms 110 and 120. In some embodiments, no more than about 5 percent, or 3 percent, or 2 percent, or 1 percent of light having a polarization along a block axis of the reflective polarizer 130 is transmitted through the polarizing beam splitter 100. In some embodiments, the combined reflectance of the reflective polarizer 130 bonded to first and second prisms 110 and 120 is less than 5 percent, or less than 3 percent, or less than 2 percent, or less than 1 percent for light polarized along a pass axis for the reflective polarizer 130.

FIG. 2 is a schematic side view of illuminator 202 including polarizing beam splitting system 204, which includes polarizing beam splitter 200 and first and second reflective components 232 and 234. Illuminator 202 further includes a lens 240 and a light source 250. Polarizing beam splitter 200, which may correspond to polarizing beam splitter 100, includes first and second prisms 210 and 220, and reflective polarizer 230. First prism 210 includes input face 212, output face 214 and first hypotenuse 216. Input face 212 has an input active area 213 and output face 214 has an output active area 215. Lens 240 has largest acceptance area 243. Second prism 220 has an imager face 224 and a second hypotenuse 226. A reflective polarizer 230 is disposed between first and second hypotenuses 216 and 226. The light source 250 produces a light beam having an envelope 252 and a central light ray 256 which defines a folded optical axis 257 having first, second, third and fourth segments, 257 a-257 d. The first reflective component 232 is disposed adjacent the polarizing beam splitter 200 opposite light source 250 and the second reflective component 234 is disposed adjacent the polarizing beam splitter 200 opposite lens 240.

The second reflective component 234 has a largest active area 236. The second reflective component 234 may be an image forming device and the largest active area 236 may be a largest image area of the image forming device. Light is emitted (by being reflected, for example) from second reflective component 234 in envelope 254. One or both of the first and second reflective components 232 and 234 may have a specular reflectance of greater than 70 percent, or greater than 80 percent, or greater than 90 percent. The first and/or second reflective components 232 and 234 may be flat or may be curved in one or more axes.

In some embodiments, second reflective component 234 is adapted to modulate light incident thereon. For example, second reflective component 234 may be an image forming device that reflects light having a spatially modulated polarization state. Second reflective component 234 may be pixelated and may produce a patterned light. Light reflected from second reflective component 234 in envelope 254 may be converging patterned light. Suitable image forming devices that can be utilized as second reflective component 234 include Liquid Crystal on Silicon (LCoS) devices. The LCoS device may be flat or may be curved in one or more axes.

The various components in FIG. 2 are shown spaced apart for clarity of illustration. However, it should be understood that the various components could be in direct contact or attached through an optically clear adhesive, for example. In some embodiments, reflective polarizer 230 is attached to one or both of first and second prisms 210 and 220 using optically clear adhesive layers. In some embodiments, lens 240 is attached to output face 214 with an optically clear adhesive. In some embodiments, light source 250 may be immediately adjacent input face 212 or may be attached to input face 212 through an optically clear adhesive layer. In some embodiments, first and/or second reflective components 232 and 234 may be attached to second prism 220 with optically clear adhesives.

Folded optical axis 257 includes first segment 257 a extending in a first direction (positive x-direction) from the light source 250 to the first reflective component 232, second segment 257 b extending in a second direction (negative x-direction) opposite the first direction, third segment 257 c extending in a third direction (negative y-direction), and fourth segment 257 d extending in a fourth direction (positive y-direction) opposite the third direction. First and second segments 257 a and 257 b are overlapping though they are shown with a small separation in FIG. 2 for ease of illustration Similarly, third and fourth segments 257 c and 257 d are overlapping though they are shown with a small separation in FIG. 2 for ease of illustration. The first and second directions are substantially orthogonal to the third and fourth directions. The first reflective component 232 is substantially perpendicular to the first segment 257 a and the second reflective component 234 is substantially perpendicular to the third segment 257 c.

Light source 250 produces a light beam having envelope 252 and this defines the input active area 213 as the area of input face 212 that is illumined with light from the light source 250 that is used by the illuminator 202. Light source 250 may either substantially not produce light outside of the envelope 252 or any light that is produced outside this envelope is at an angle that it escapes from the illuminator without entering lens 240.

At least a portion of the light from light source 250 is, in sequence, transmitted through the first prism 210, transmitted through the reflective polarizer 230, transmitted through the second prism 220, reflected from the first reflective component 232, transmitted back through the second prism 220, reflected from the reflective polarizer 230, transmitted through the second prism 220 and is incident on second reflective component 234, reflected from second reflective component 234, transmitted through second prism 220 and reflective polarizer 230 and first prism 210, and finally exits the illuminator through lens 240. This is illustrated in FIG. 2 for central light ray 256. In some embodiments, first reflective component 232 includes a polarization rotator, which may be a quarter wave retarder. Light from the light source 250 that has a polarization along the pass axis of reflective polarizer 230 will be transmitter through the reflective polarizer 230 and then reflect from first reflective component 232 back towards the reflective polarizer 230. In embodiments in which first reflective component 232 includes a quarter wave retarder, such light passes twice through the quarter wave retarder when it reflects back toward the reflective polarizer 230. This light then has a polarization substantially orthogonal to the pass axis of the reflective polarizer 230 and so reflects from the reflective polarizer 230 toward second reflective component 234 which may emit (e.g., reflect) spatially modulated light back toward reflective polarizer 230. The spatially modulated light may have a polarization that is spatially modulated. The portion of the spatially modulated light having a polarization along the pass axis of reflective polarizer 230 will pass through the reflective polarizer 230 as an imaged light, exit first prism 210 through output active area 215 and exit the illuminator through the lens 240.

The illuminator 202 allows an image to be projected by directing a light beam (in envelope 252) through a folded light path illuminator 202 onto an imaging forming device (second reflective component 234), and reflecting a converging patterned light (in envelope 254) from the image forming device. The step of directing a light beam through the folded light path illuminator 202 includes directing light to the first reflective component 232 through the polarizing beam splitter 200, reflecting at least some of the light back towards the polarizing beam splitter 200, and reflecting at least some of the light from the polarizing beam splitter 200 towards the image forming device. At least a portion of the converging patterned light is transmitted through the polarizing beam splitter 200 and through lens 240.

Light from light source 250 illuminates a maximum area of second reflective component 234 after the light is reflected from the first reflective component 232 and the reflective polarizer 230. This maximum area may be equal to the largest active area 236. Alternatively, the largest active area 236 may be a largest area of second reflective component 234 that is reflective. For example, second reflective component 234 may be an image forming device that has a largest image area. Any light incident on the image forming device outside the largest image area may not be reflected towards lens 240. In this case, the largest active area 236 would be the largest image area of the image forming device. The largest active area 236 defines the output active area 215 on output face 214 and largest acceptance area 243 of lens 240 since light is reflected from the largest active area 236 towards lens 240 in envelope 254 which illuminates the output face 214 substantially only in the output active area 215 and illuminates the lens 240 substantially only in the largest acceptance area 243. Illuminator 202 is configured such that light in envelope 254 that is reflected from the second reflective component 234 and that passes through the lens 240 is convergent between the second reflective component 234 and the lens 240. This results in a largest active area 236 that is smaller than the output active area 215 which is smaller than the largest active area 236.

In some embodiments, the input active area 213 and/or the output active area 215 are less than about 60 percent, or less than about 50 percent (i.e., less than about half), or less than about 40 percent, or less than about 35 percent of the largest active area 236, which may be a largest image area. In some embodiments, the largest surface area of input face 212 (the total area of input face 212) is less than about half the largest image area. In some embodiments, the largest surface area of the output face 214 (the total area of output face 214) is less than about half the largest image area.

Light source 250, or any of the light sources of the present description, may include one or more substantially monochromatic light emitting elements. For example, light source 250 may include red, green and blue light emitting diodes (LEDs). Other colors, such as cyan and yellow may also be included. Alternatively, or in addition, broad spectrum (e.g., white or substantially white) light sources may be utilized. In some embodiments, the light source 250 includes a blue emitter and a phosphor. In some embodiments, the light source 250 includes an integrator that may be utilized to combine light from discrete light sources (e.g., the integrator may combine light from red, green and blue LEDs). The light source 250 may include a polarizing element such that light having substantially a single polarization state is directed into first prism 210 towards reflective polarizer 230. In some embodiments, light source 250 may be or may include one or more of an LED, an organic light emitting diode (OLED), a laser, a laser diode, an incandescent lighting element, and an arc lamp Light source 250 may also include a lens, such as a condenser lens, in addition to lighting emitting element(s) such as LED(s).

In some embodiments, the first or second prisms may have one or more curved faces to provide a desired optical power. FIG. 3 is a side view of illuminator 302 including polarizing beam splitting system 304, which includes polarizing beam splitter 300 and first and second reflective components 332 and 334. Illuminator 302 further includes a lens 340, which may be an element of a projection lens 344, and a light source 350. Polarizing beam splitter 300 includes first and second prisms 310 and 320, and reflective polarizer 330. First prism 310 includes input face 312 and output face 314. Second prism 320 has an imager face 324 and a second face 322. A reflective polarizer 330 is disposed between first and second hypotenuses of the first and second prisms 310 and 320.

Second prism 320 includes one or more components 360 and one or more components 362 which may be attached to body 364 of second prism 320 through one or more optically clear adhesives, for example. In some embodiments, components 360 and 362 may be separated (e.g., with an air gap) from body 364. In some embodiments, body 364 may be a right triangular prism. In some embodiments, one or both of components 360 and 362 may be formed integrally with body 364, by injection molding, for example, or by any other suitable forming process. In some embodiments, input face 312 and/or output face 314 may similarly include one or more components with a curved surface(s) attached to a body of the first prism 310 or may include a curved surface formed integrally with first prism 310.

In the illustrated embodiments, first reflective component 332 is a reflective coating applied to second face 322 of second prism 320 and a quarter wave retarder 365 is disposed between body 364 and components 362. In other embodiments, component 362 may be formed integrally with body 364 and a quarter wave retarder may be applied to second face 322 and a reflective coating may then be applied to the quarter wave retarder.

Light source 350 produces central light ray 356 and outer envelope light rays 352 a and 352 b. Light ray 352 b (and similarly for light ray 352 a and central light ray 356) is emitted by light source 350 having a polarization along the pass axis of reflective polarizer 330. Light ray 352 b, in sequence, passes through first prism 310, pass through reflective polarizer 330, passes through body 364 of second prism 320, passes through quarter wave retarder 365, passes through components 362, is reflected by first reflective component 332, passes back through components 362, and then passes back through quarter wave retarder 365 and back through body 364 towards reflective polarizer 330. Since the light ray 352 b has made two passes through the quarter wave retarder, it has a polarization substantially orthogonal to the pass axis of reflective polarizer 330. Light ray 352 b therefore reflects from reflective polarizer 330, passes through body 364 and components 360 and is then reflected from second reflective component 334 back through components 360 and body 364 towards reflective polarizer 330. Second reflective component 334 may be an image forming device that spatially modulates the polarization of light reflected from the second reflective component 334. In such cases, a portion of the light reflected from the second reflective component 334 may have a polarization along the pass axis of the reflective polarizer 330. This is the case for light ray 352 b which passes through reflective polarizer 330 after reflecting from second reflective component 334. Light ray 352 b then passes through first prism 310 and exits through output face 314. Light ray 352 b then passes through projection lens 344 and then exits the illuminator 302.

In some cases, it may be useful to have the light source adjacent a larger prism rather than a smaller prism. An exemplary embodiment is illustrated in FIG. 4 which is a side view of illuminator 402 including a first prism 410, a second prism 420, a reflective polarizer 430, a light source 450, a lens 440 which is an element of a projection lens 444, and a lens 462 disposed between light source 450 and face 422 of second prism 420. Second prism 420 also has face 424 and includes components 460 which may be formed integrally with a body of the second prism 420 or may be attached to the body of the second prism 420 with an optically clear adhesive, for example. The lens 462 has a first and second surface 466 and 468, respectively. Light source 450 may correspond to any of the light sources described elsewhere herein.

In some embodiments, as shown in FIG. 4B, the first surface 466 includes a quarter wave retarder 465 disposed on the first surface 466 and a reflector 432 (e.g., a reflective coating) disposed on the quarter wave retarder 465. In some embodiments, a quarter wave retarder may be disposed adjacent (possibly, but not necessarily immediately adjacent) first surface 466 and a reflector may be disposed adjacent (possibly, but not necessarily immediately adjacent) the quarter wave retarder opposite the first surface 466. The reflector includes an aperture 433 over the emitting face of the light source 450 such that light emitted from the light source 450 passes into the lens 462. The aperture may optionally extend into the quarter wave retarder 465. A reflective polarizer 439 may be attached to the second surface 468. In alternate embodiments, the reflective polarizer 439 may be adjacent but not necessarily immediately adjacent second surface 468.

The arrangement of reflector 432, quarter wave retarder 465 and reflective polarizer 439 provide a polarization converter for light source 450. Light incident on reflective polarizer 439 having a polarization along the pass direction for the reflective polarizer 439 exits lens 462 into second prism 420. Light having the orthogonal polarization is reflected from reflective polarizer 439, passes through the lens 462 and through quarter wave retarder 465, then reflects from reflector 432 and passes back through quarter wave retarder 465 towards reflective polarizer 439. Since the light has made two passes through the quarter wave retarder 465, it now it is polarized along the pass axis of reflective polarizer 439 and so it passes through the reflective polarizer 439 into second prism 420.

FIGS. 5A and 5B are a side views of illuminator 502 including first and second prisms 510 and 520, a reflective polarizer 530 disposed between hypotenuses of the first and second prisms 510 and 520, first and second reflective components 532 and 534, a lens 540, which may be an element of a projection lens, and a light source 550. First prism 510 includes input surface 512 and output surface 514. Second prism 320 has a first surface 522 and a second surface 524. A central light ray emitted by light source 550 defines a folded optical axis 557 in a similar way that central light ray 256 defines folded optical axis 257. The folded optical axis 557 has a length d1 between the input surface 512 and the reflective polarizer 530, a length d2 between the first reflective component 532 and the reflective polarizer 530, a length d3 between the second reflective component 534 and the reflective polarizer 530, and a length d4 between the output surface 514 and the reflective polarizer 530. In some embodiments, one or both of d1 and d4 are less than the lesser of d2 and d3, or less than 0.9 times the lesser of d2 and d3, or less than 0.85 times the lesser of d2 and d3. In some embodiments, the reflective polarizer 530 has a largest lateral dimension of d5 (described further elsewhere herein) and one or both of d1 and d4 is less than d5/4, or less than 0.2 times d5 or less than 0.15 times d5. Polarizing beam splitter 500 includes first and second prisms 510 and 520 and reflective polarizer 530.

In other embodiments, lens 540 is replaced by a spatial light modulator and second reflective component 534 is replaced by a lens. In such embodiments, light from light source 550 is reflected from reflective polarizer 530 towards the spatial light modulator (in the position of lens 540) whereupon it is reflected as imaged light through the reflective polarizer and through the lens (in positon of the reflective component 534). Also in such embodiments, the first reflective component 532 may be omitted. In some embodiments, the geometry of the polarizing beam splitter 530 is unchanged from that described in connection with FIGS. 5A-7 when the lens is replaced by a spatial light modulator and the second reflective component is replaced by a lens.

In some embodiments, first reflector 532 is a tilted dichroic plate. A tilted dichroic plate is a reflector that includes at least one dichroic reflector tilted with respect to the optical axis 557 so that the dichroic reflector is neither perpendicular nor parallel to the optical axis 557. The tilted dichroic plate may include dichroic reflector(s) laminated together such that an outer surface of the tilted dichroic plate is perpendicular to the optical axis 557 but the dichroic reflector(s) are not. In some embodiments, the tilted dichroic reflector includes a plurality of dichroic reflectors tilted relative to one another. A tilted dichroic plate may be used to account for offsets of colored light sources from the optical axis 557. In some embodiments, the tilted dichroic plate includes a first dichroic reflector capable of reflecting a first color light and transmitting other color light; and a second reflector capable of reflecting a second color light. The first dichroic reflector and the second reflector are each tilted such that the first and the second color light are both reflected back into prism 520 along folded optical axis 557 as a combined color light beam. In some embodiments, the tilted dichroic plate includes a first dichroic reflector capable of reflecting a first color light and transmitting a second and a third color light; a second dichroic reflector capable of reflecting the second color light and transmitting the third color light; and a third reflector capable of reflecting the third color light. The first dichroic reflector, the second dichroic reflector, and the third reflector are each tilted such that the first, the second, and the third color light are each reflected back into prism 520 along folded optical axis 557 as a combined color light beam. In some embodiments, the reflective polarizer 530 is replaced with a plurality of dichroic reflective polarizers tilted relative to each other. Such tilted dichroic plates and tilted dichroic reflective polarizers are described in PCT Pub. No. WO 2013/062930 (Schardt et al.) and U.S. Pat. Appl. Pub. Nos. 2013/0169937 (Ouderkirk et al.), 2013/0169893 (Ouderkirk et al.), 2013/0169894 (Ouderkirk et al.) and 2014/0253849 (Poon et al.), for example, which are hereby incorporated herein by reference to the extent that they do not contradict the present description.

Other lengths that may be useful in describing the geometry of illuminator 502 are illustrated in FIG. 5B. The folded optical axis 557 has a length d1′ between the light source 550 and the reflective polarizer 530, a length d2′ between second surface 524 and the reflective polarizer 530, a length d3′ between first surface 522 and the reflective polarizer 530, and a length d4′ between the lens 540 and the reflective polarizer 530. In some embodiments, one or both of d1′ and d4′ are less than the lesser of d2 and d3, or less than 0.9 times the lesser of d2 and d3, or less than 0.85 times the lesser of d2 and d3. In some embodiments, one or both of d1′ and d4′ are less than a lesser of d2′ and d3′, or less than 0.9 times the lesser of d2′ and d3′, or less than 0.85 times the lesser of d2′ and d3′. In some embodiments, the reflective polarizer 530 has a largest lateral dimension of d5 and one or both of d1′ and d4′ is less than d5/4, or less than 0.2 times d5 or less than 0.15 times d5.

As illustrated in FIG. 5C, the reflective polarizer 530 may have a largest lateral dimension d5. In cases where the reflective polarizer is rectangular shaped with sides having dimensions L and W, the largest lateral dimension d5 of the reflective polarizer 530 is given by d5=(L²+W²)^(1/2). The largest lateral dimension d5 may be greater than four or five times d1 and/or may be greater than four or five times d4. The largest lateral dimension d5 may be greater than four or five times d1′ and/or may be greater than four or five times d4′. In some embodiments, the second prism is a right triangular prism and L and W are substantially equal. The lengths d2′ and d3′ may then be approximately L (or W) divided by 2√2 and d5 may be approximately equal to four times d2′ or approximately equal to four times d3′.

Any of the relative relationships described between any of d1, d2, d3, d4, d5 may also hold if any one or more of d1, d2, d3, d4 are replaced with d1′, d2′, d3′, d4′, respectively Similarly, any of the relative relationships described between any of d1′, d2′, d3′, d4′, d5 may also hold if any one or more of d1′, d2′, d3′, d4′ are replaced with d1, d2, d3, d4, respectively.

In some embodiments, the folded optics design allows a first prism to have a substantially smaller volume than a second prism. In other embodiments, the first and second prisms may have substantially the same volume, and the folded optics design may be used with a lens having a small acceptance area and/or with a light source having a small emitting area. This is illustrated in FIG. 6 which is a side view of illuminator 602 including first and second prism 610 and 620, reflective polarizer 630 disposed between the first and second prism 610 and 620, first and second reflective components 632 and 634, and light source 650 having an emitting area 651. First prism 610 includes first and second surfaces 612 and 614, and second prism 620 includes first and second surfaces 622 and 624. Illuminator 602 further includes lens 640 which may optionally be bonded to first prism 610 with an optically clear adhesive layer 641. First reflective component 632 may include a quarter wave retarder as described elsewhere herein, and second reflective component 634 may be an image forming device as described elsewhere herein and may emit a converging patterned light towards lens 640. Emitting area 651 of light source 650 and/or and acceptance area of lens 640 may be less than 60 percent, or less than 50 percent, or less than 40 percent, or less than 35 percent of a largest active area or of a largest image area of second reflective component 634.

FIG. 7 is a side view of illuminator 702 including a light source 750, a reflective polarizer 730 in optical communication with the light source 750, and a lens 740 in optical communication with the reflective polarizer. The reflective polarizer 730 defines a smallest imaginary rectangular parallelepiped 770 entirely containing the reflective polarizer 730 and having a surface (surfaces 772 and 774) perpendicular to a central light ray 756 emitted by the light source 750. At least a portion of the light source 750 or at least a portion of the lens 740 is disposed inside the imaginary rectangular parallelepiped 770. In some embodiments, at least a portion of the light source 750 and at least a portion of the lens 740 are disposed inside the imaginary rectangular parallelepiped 770. In some embodiments, all or substantially all of the light source 750 or all or substantially all of the lens 740 is disposed inside the imaginary rectangular parallelepiped 770. In some embodiments, all or substantially all of the light source 750 and all or substantially all of the lens 740 is disposed inside the imaginary rectangular parallelepiped 770.

In some embodiments, lens 740 is an element of a projection lens. In some embodiments, the illuminator 702 also includes an image forming device 734 substantially perpendicular to a surface (surfaces 772 and 774) of the imaginary rectangular parallelepiped 770. In some embodiments, illuminator 702 includes first and/or second prisms, corresponding to the first and second prisms of any of the embodiments described herein, and/or includes a reflective component proximate surface 772 as, for example, shown in any of FIGS. 2-5B.

The illuminators of the present description are useful, for example, when compact projection is desired. In some aspects of the present description, a head mounted system, such as a head mounted display, is provided. Head mounted systems are described, for example, in PCT publication WO 2015/034801 (Ouderkirk) and in U.S. Prov. App. No. 61/977171 (Ouderkirk et al.), each of which is incorporated herein by reference to the extent that they do not contradict the present description.

FIG. 8 is a schematic illustration of head mounted system 801 including unit 809 mounted to a frame 880 that includes first and second lenses 882 and 884. Unit 809 is disposed to provide to and/or receive light from first lens 882. In some embodiments, a second unit is mounted to frame 880 to provide to and/or receive light from first lens 882. Unit 809 may be or may include any of the illuminators, polarizing beam splitters, or polarizing beam splitting systems of the present description.

The head mounted system 801 may include an eye monitoring system which may be include in unit 809. The system may monitor the diameter and position of the pupil with an imaging sensor and processor via the first lens 882 positioned in front of the eye. The first lens 882 may include partially transparent reflector either adjacent to or embedded in it, where the reflector produces an image of the pupil on the sensor. The system may quantify fatigue and cognitive processing load of the user of the system based on pupillary response with considerations of ambient light conditions and may be personalized to the user based on historical data. The quantified information may be reported and visualized via a software application, such as a workforce management program or smartphone application.

These attributes of the eye that the eye monitoring system can detect may include one or more of the following: the viewing direction of the eye, diameter and changes in the diameter of the pupil, blinking of the eyelids, the eye tracking objects, and saccade movement. Eye tracking parameters may include velocity of the eye rotation and lag or phase between movement of an object and movement of the eye. Saccade movement may include duration, velocity, and pattern of the movement.

In some embodiments, the head mounted system 801 includes a camera (e.g., a red-green-blue (RGB) camera or an infrared (IR) camera) that may be included in unit 809 and that can capture an image of the eye. An IR camera can be used to determined ambient light conditions since the average IR luminance of the eye image is indicative of the ambient light levels. In some embodiments, the head mounted system 801 is adapted to implement a computer vision algorithm running on an embedded system which may be included in unit 809.

In some embodiments, the head mounted system includes an eye tracking system adapted to detect changes in pupil size and use that information to quantify user fatigue and cognitive processing load. In some embodiments, the head mounted system 801 is adapted (e.g., using an algorithm running on an embedded processor) to implement one or more or all of the following steps:

Step 1: Capture a grayscale image of the eye.

Step 2: Filter out noise (e.g. using a Gaussian filter).

Step 3: Calculate gradient magnitude and direction for each pixel in the image of the eye.

Step 4: Identify pixels with higher gradient magnitudes (these are likely to be an edge of an object).

Step 5: Identify edges by, for example, connecting the pixels identified in the previous step according to the Helmholtz Principle of human visual perception.

Step 6: Compare edge line segments to the equation of an ellipse or other shape defined by a polynomial equation. The smallest ellipse-like shape can be identified as the pupil. The area of the iris can also be determined and may be used to improve accuracy. Other elliptical shapes that may be in the image, such as glint, can be eliminated.

Step 7: Calculate the pupil size (e.g., diameter or area) based on the line fitting done previously and the distance between the eye and the camera.

Step 8: Determine and apply an adjustment factor to the calculated pupil size to account for ambient light conditions. Ambient light conditions can be determined using an additional sensor included in the head mounted system or via luminance analysis of the image captured.

Step 9: Save the adjusted pupil size in a database, which may be a secure database, for historical comparisons and analysis of cognitive processing load and fatigue levels. Such a database could conceivably hold other biological data (such as heart rate, skin conductivity, electroencephalographs (EEGs), etc.) which could be used in a sensor fusion algorithm to further analyze the user's mental state. The pupil size may be recorded as a function of time and may be stored as a time-series (a sequence of data points made over time).

The method of fatigue and cognitive load analysis can utilize historical data to determine if the current levels exceed a threshold. This threshold can vary from person to person and may be determined using a machine learning algorithm once enough historical data has been gathered by the system and procedure described above. If the threshold of fatigue level or cognitive processing load is exceeded, a software application can be utilized to alert the user or a central office manager, for example. Furthermore, the historical data (e.g., time-series of pupil diameters) can be visualized (e.g., in a line graph of pupil size over time) in a software application for a quick indication of current cognitive states. The eye tracking system may also track the movement of the eye by storing the location of the pupil in the image captured by the system over time. Including this location of the pupil in the time-series could provide information into how quickly the eye is moving, which provides another way that fatigue could be measured since slower moving eyes are more fatigued than quickly moving eyes.

The following is a list of exemplary embodiments.

Embodiment 1 is a polarizing beam splitting system comprising:

a reflective polarizer,

a first prism having a first volume and comprising:

-   -   a first face;     -   a second face adjacent the first face, an angle between the         first and second faces substantially equal to 90 degrees; and     -   a first hypotenuse opposite the angle;

a light source disposed adjacent the first face;

an image forming device disposed adjacent the second face;

a second prism having a second volume, the second prism a right triangular prism having third and fourth faces and having a second hypotenuse, the second hypotenuse disposed adjacent the first hypotenuse, the first the first and second hypotenuses having substantially equal surface areas, the third face opposite the first face and substantially parallel with the first face, the fourth face opposite the second face and substantially parallel with the second face;

wherein the reflective polarizer is disposed between the first and second hypotenuses, and wherein the first volume is no greater than half the second volume.

Embodiment 2 is the polarizing beam splitting system of Embodiment 1, wherein the first face has a largest area that is less than half of a largest area of the third face and that is less than half a largest area of the fourth face.

Embodiment 3 is the polarizing beam splitting system of Embodiment 1, wherein the second face has a largest area that is less than half of a largest area of the third face and that is less than half of a largest area of the fourth face.

Embodiment 4 is the polarizing beam splitting system of Embodiment 1, wherein the each of a largest area of the first face and a largest area of the second face is less than half of a lesser of a largest area of the third face and a largest area of the fourth face.

Embodiment 5 is the polarizing beam splitting system of Embodiment 1, further comprising a portion extending from the first and second sides, wherein the first hypotenuse comprises a major surface of the portion.

Embodiment 6 is the polarizing beam splitting system of Embodiment 1, wherein the reflective polarizer is a polymeric multilayer reflective polarizer, a wire grid polarizer, a MacNeille reflective polarizer, or a cholesteric reflective polarizer.

Embodiment 7 is the polarizing beam splitting system of Embodiment 1, wherein the reflective polarizer is a polymeric multilayer reflective polarizer.

Embodiment 8 is the polarizing beam splitting system of Embodiment 1 being centered on an optical axis defined by an optical path of a central light ray emitted by the light source, the optical axis having a length d1 between the first face and the reflective polarizer, the optical axis having a length d3′ between the fourth face and the reflective polarizer, d1 being less than d3′.

Embodiment 9 is the polarizing beam splitting system of Embodiment 8, wherein d1 is less than 0.9 times d3′.

Embodiment 10 is the polarizing beam splitting system of Embodiment 8, wherein the reflective polarizer has a largest lateral dimension of d5 and d1 is less than d5/4.

Embodiment 11 is the polarizing beam splitting system of Embodiment 10, wherein d1 is less than 0.2 times d5.

Embodiment 12 is a polarizing beam splitting system adapted to receive light from a light source and centered on a folded optical axis defined by an optical path of a central light ray emitted by the light source, the polarizing beam splitting system comprising:

an input surface substantially perpendicular to the optical axis, light entering the polarizing beam splitting system by passing through the input surface;

a reflective polarizer having a largest lateral dimension d5, the optical axis having a length d1 between the input surface and the reflective polarizer;

an output surface substantially perpendicular to the optical axis, the output surface having a length d3′ between the output surface and the reflective polarizer, light exiting the polarizing beam splitting system by passing through the output surface; and

an imager face substantially perpendicular to the optical axis, the optical axis having a length d4 between the imager face and the reflective polarizer,

wherein one or both of d1 and d4 are less than d5/4.

Embodiment 13 is the polarizing beam splitting system of Embodiment 12, wherein d1 is less than d5/4.

Embodiment 14 is the polarizing beam splitting system of Embodiment 12, wherein d4 is less than d5/4.

Embodiment 15 is the polarizing beam splitting system of Embodiment 12, wherein each of d1 and d4 is less than d5/4.

Embodiment 16 is the polarizing beam splitting system of Embodiment 12, wherein one or both of d1 and d4 are less than 0.2 times d5.

Embodiment 17 is the polarizing beam splitting system of Embodiment 12, wherein one or both of d1 and d4 are less than d3′.

Embodiment 18 is the polarizing beam splitting system of Embodiment 12, wherein d1 is less than d3′.

Embodiment 19 is the polarizing beam splitting system of Embodiment 12, wherein d4 is less than d3′.

Embodiment 20 is the polarizing beam splitting system of Embodiment 12, wherein each of d1 and d4 is less than d3′.

Embodiment 21 is the polarizing beam splitting system of Embodiment 20, wherein each of d1 and d4 is less than 0.9 times d3′.

Embodiment 22 is a polarizing beam splitting system adapted to receive light from a light source and centered on a folded optical axis defined by an optical path of a central light ray emitted by the light source, the polarizing beam splitting system comprising:

-   -   an input surface substantially perpendicular to the optical         axis, light entering the polarizing beam splitting system by         passing through the input surface;     -   a reflective polarizer, the optical axis having a length d1         between the input surface and the reflective polarizer;     -   a first reflective component substantially perpendicular to the         optical axis, the optical axis having a length d2 between the         first reflective component and the reflective polarizer, the         first reflective component being a tilted dichroic plate;     -   a second reflective component substantially perpendicular to the         optical axis, the optical axis having a length d3 between the         second reflective component and the reflective polarizer; and     -   an output face substantially perpendicular to the optical axis,         light exiting the polarizing beam splitting system by passing         through the output surface, the optical axis having a length d4         between the output surface and the reflective polarizer, one or         both of d1 and d4 being less than a lesser of d2 and d3.

Embodiment 23 is the polarizing beam splitting system of Embodiment 22, wherein d4 is less than the lesser of d2 and d3.

Embodiment 24 is the polarizing beam splitting system of Embodiment 22, wherein d1 is less than the lesser of d2 and d3.

Embodiment 25 is the polarizing beam splitting system of Embodiment 22, wherein both d1 and d4 are less than the lesser of d2 and d3.

Embodiment 26 is the polarizing beam splitting system of Embodiment 22, wherein one or both of d1 and d4 less than 0.9 times the lesser of d2 and d3.

Embodiment 27 is the polarizing beam splitting system of Embodiment 22, wherein the reflective polarizer has a largest lateral dimension of d5, one or both of d1 and d4 being less than d5/4.

Embodiment 28 is the polarizing beam splitting system of Embodiment 27, wherein each of d1 and d4 is less than d5/4.

Embodiment 29 is the polarizing beam splitting system of Embodiment 27, wherein one or both of d1 and d4 is less than 0.2 times d5.

Embodiment 30 is the polarizing beam splitting system of Embodiment 22, wherein the second reflective component is adapted to modulate light incident thereon.

Embodiment 31 is the polarizing beam splitting system of Embodiment 22, wherein the second reflective component is pixelated.

Embodiment 32 is the polarizing beam splitting system of Embodiment 22, wherein an angle between the reflective polarizer and the optical axis is between 40 to 60 degrees.

Embodiment 33 is the polarizing beam splitting system of Embodiment 22, wherein the first reflective component has a specular reflectance greater than 80%.

Embodiment 34 is the polarizing beam splitting system of Embodiment 22, wherein the second reflective component has a specular reflectance greater than 80%.

Embodiment 35 is a polarizing beam splitting system adapted to receive light from a light source and centered on a folded optical axis defined by an optical path of a central light ray emitted by the light source, the polarizing beam splitting system comprising:

-   -   an input surface substantially perpendicular to the optical         axis, light entering the polarizing beam splitting system by         passing through the input surface;     -   a reflective polarizer having a largest lateral dimension d5,         the optical axis having a length d1 between the input surface         and the reflective polarizer;     -   a first reflective component substantially perpendicular to the         optical axis, the optical axis having a length d2 between the         first reflective component and the reflective polarizer, the         first reflective component being a tilted dichroic plate;     -   a second reflective component substantially perpendicular to the         optical axis, the optical axis having a length d3 between the         second reflective component and the reflective polarizer; and     -   an output face substantially perpendicular to the optical axis,         light exiting the polarizing beam splitting system by passing         through the output surface, the optical axis having a length d4         between the output surface and the reflective polarizer,     -   wherein one or both of d1 and d4 are less than d5/4.

Embodiment 36 is the polarizing beam splitting system of Embodiment 35, wherein d1 is less than d5/4.

Embodiment 37 is the polarizing beam splitting system of Embodiment 35, wherein d4 is less than d5/4.

Embodiment 38 is the polarizing beam splitting system of Embodiment 35, wherein each of d1 and d4 is less than d5/4.

Embodiment 39 is the polarizing beam splitting system of Embodiment 35, wherein one or both of d1 and d4 are less than 0.2 times d5.

Embodiment 40 is the polarizing beam splitting system of Embodiment 35, wherein one or both of d1 and d4 are less than a lesser of d2 and d3.

Embodiment 41 is the polarizing beam splitting system of Embodiment 40, wherein d1 is less than the lesser of d2 and d3.

Embodiment 42 is the polarizing beam splitting system of Embodiment 40, wherein d4 is less than the lesser of d2 and d3.

Embodiment 43 is the polarizing beam splitting system of Embodiment 40, wherein each of d1 and d4 is less than the lesser of d2 and d3.

Embodiment 44 is the polarizing beam splitting system of Embodiment 43, wherein each of d1 and d4 is less than 0.9 times the lesser of d2 and d3.

Embodiment 45 is the polarizing beam splitting system of Embodiment 35, wherein the first reflective component has a specular reflectance greater than 80%.

Embodiment 46 is the polarizing beam splitting system of Embodiment 35, wherein the second reflective component has a specular reflectance greater than 80%.

Embodiment 47 is a head mounted system comprising the polarizing beam splitting system of any of Embodiments 1 to 46, the head mounted system configured to provide images to a viewer.

Embodiment 48 is the head mounted system of Embodiment 47 further comprising an eye tracking system.

Embodiment 49 is the head mounted system of Embodiment 48, wherein the eye tracking system is adapted to determine pupil size.

Embodiment 50 is the head mounted system of Embodiment 49, wherein the eye tracking system is adapted to record a time-series of pupil diameters.

Embodiment 51 is the head mounted system of Embodiment 50, wherein the eye tracking system is adapted to record a time-series of pupil locations.

Descriptions for elements in figures should be understood to apply equally to corresponding elements in other figures, unless indicated otherwise. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations can be substituted for the specific embodiments shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this disclosure be limited only by the claims and the equivalents thereof. 

What is claimed is:
 1. A polarizing beam splitting system comprising: a first prism comprising a first face and a first peak opposite the first face; a second prism disposed adjacent the first face of the first prism and comprising: opposing facets meeting at a second peak opposite the first peak; and extensions extending from the facets along the first face of the first prism, each extension comprising opposing substantially parallel major surfaces; and a reflective polarizer disposed between the first and second prisms.
 2. The polarizing beam splitting system of claim 1, wherein the second prism comprises a substantially planar major surface opposite the second peak, the substantially planar major surface comprising a major surface of each extension.
 3. The polarizing beam splitting system of claim 2, wherein the substantially planar major surface of the second prism has a substantially same total area as the first face of the first prism.
 4. The polarizing beam splitting system of claim 3, wherein the substantially planar major surface and the first prism are substantially coextensive with the reflective polarizer.
 5. The polarizing beam splitting system of claim 4, wherein the second prism has a volume no greater than about half a volume of the first prism.
 6. The polarizing beam splitting system of claim 1, wherein the first prism is a right triangular prism, and the opposing facets of the second prism define an angle of about 90 degrees therebetween.
 7. The polarizing beam splitting system of claim 6, wherein the second prism has a volume no greater than about half a volume of the first prism.
 8. The polarizing beam splitting system of claim 1, wherein the second prism has a volume no greater than about 40 percent of a volume of the first prism.
 9. The polarizing beam splitting system of claim 1, wherein each of the opposing facets has a substantially same area.
 10. An illuminator comprising the polarizing beam splitting system of claim 1 and a light source, the polarizing beam splitting system being disposed to receive light from the light source and being centered on a folded optical axis defined by an optical path of a central light ray emitted by the light source, each of the opposing facets of the second prism being substantially perpendicular to the folded optical axis.
 11. The illuminator of claim 10, wherein the first prism comprises opposing faces meeting at the first peak, each of the opposing faces being substantially perpendicular to the folded optical axis.
 12. A head mounted system comprising the polarizing beam splitting system of claim 1, the head mounted system configured to provide images to a viewer.
 13. The head mounted system of claim 12 further comprising an eye tracking system adapted to determine pupil size, wherein the eye tracking system is adapted to record a time-series of pupil diameters and a time-series of pupil locations. 